A hybrid vehicle or an electric vehicle is equipped with an assembled battery including multiple secondary batteries (unit batteries) connected in series. In such an assembled battery, it is necessary to detect the voltage of each secondary battery cell individually for calculating the capacity of each secondary battery cell and protection management of the secondary battery cell. It is noted that an assembled battery for such an application includes a significantly large number of secondary batteries connected in series. Therefore, the electric potential becomes high in a specific connection position between secondary batteries in the assembled battery. Thus, the voltage detection device for the secondary battery is applied with high voltage.
For example, JP-A-2008-145180 discloses a voltage detection circuit including an operational amplifier, a first capacitor, a second capacitor, a discharge circuit, and switch devices. The first capacitor has one end was connected to the inverting input terminal of the operational amplifier. The second capacitor and the discharge circuit are connected between the inverting input terminal of the operational amplifier and the output terminal of the operational amplifier. The switch devices are respectively connected between the terminal of each of the unit batteries and the other end of the first capacitor. In the configuration of JP-A-2008-145180, the switch device and the discharge circuit between the positive terminal of the unit battery and the first capacitor are activated to charge the first capacitor. Thereafter, in the state where the discharge circuit is deactivated, the switch device between the negative terminal of the unit battery and the first capacitor is activated, instead of the switch device, thereby to detect the voltage of the unit battery.
JP-A-2001-56350 discloses a voltage detection circuit including a capacitor group constructed of multiple capacitors connected in series, correspondingly to the unit batteries. In the configuration of JP-A-2001-56350, each switch device provided in the path between the unit battery and the capacitor is closed to charge the corresponding capacitor by applying the voltage of the unit battery all at once. Thereafter, voltage detection is implemented from the low-voltage side capacitor in order. When the voltage detection of one of the capacitors is completed, the electric charge of the one capacitor is discharged, thereby to successively reduce the electric potential of an undetected capacitor in this way.
The voltage detection circuits disclosed in JP-A-2008-145180 and JP-A-2001-56350 include the operational amplifier and the microcomputer with low-voltage configurations. However, semiconductor switch devices employed in these voltage detection circuits are applied with still high voltage. Therefore, a low-voltage process, such as a 5V system or a 3.3V system cannot be employed in these voltage detection circuits. Consequently, a high-voltage transistor is requisite in such a voltage detection circuit to result in increase in a layout area for the circuit. Thus, manufacturing cost for the voltage detection circuit may increase.